1. Field of the Invention
The present invention relates to a parent material for wooden patterns to be used for the production of cast parts in car, machine and other parts and to a method of manufacturing it.
2. Description of the Prior Art
Previously the wooden model or pattern for making a sand cast mold for the production of cast parts has been made by using such natural timbers as Japanese white pine, red pine and magnolia after desiccating for a long years, and wax and other materials have been applied for moisture-proofing over the surface. In the case of mass production of cast mold a model (original pattern of cast mold) made of aluminium, brass or other metal or resin has been used in place of wooden pattern. In view of mechanical processibility and economy ABS foam, polyethylene foam, polypropylene foam and other synthestic resin foams made of thermosetting, synthetic resins have been utilized extensively as the parent material for making such wooden patterns, and such thermosetting, synthetic resin foams as of epoxy, phenol and polyurethane resins have also been used.
When natural timbers are used as the parent material for wooden patterns, they are required to be desiccated for a long period (longer than 10 years), and still the produced pattern shows distortion by swelling, contraction and others due to moisture absorption after some 6 months. Since natural timbers show direction of grain, which is the causative factor of the distortion, wood plates are layered alternately in regard to the direction of grain for preventing the distortion, but it has not been able to prevent it so far. It is also noted that the presence of grain and knot makes natural wood material heterogeneous, and it is a cause of such a shortcoming of difficulty in cutting during processing. Natural timbers also show limitation in the surface hardness being lower in comparison with metals, and they show relatively low contraction resistance and subsequently defectively inferior durability.
When aluminium, brass and other metals are used for making parent material for wooden patterns, processibility is satisfactory and excellent pattern (model) in regard to distortion and durability can be produced, but it is defective in being expensive.
Such above-stated synthetic resins as of ABS. polyethylene, polypropylene, epoxy, phenol and urethane are used in the foamed state for making the parent material for wooden patterns. The reason of using resin foams is that in processing of synthetic resins thermoplastic resins become sticky due to their melting by frictional heat of cutting blade during cutting, while thermosetting resins are so hard and fragile that cutting is difficult to do, and foamed resins have been employed for easier cutting. However, said synthetic resin foams often vibrate finely during cutting posing a problem of difficulty in high speed cutting. In addition the foamed thermoplastic resins are molded for making the parent material after melting similarly as with unfoamed ones, and they are molten by heating, extruded to fill forming mold and solidified by cooling, and during these processes the resins show internal stress that remains and works to move in the backward direction to the flowing direction. Accordingly the internal stress is liable to lose balancing during cutting the parent material for making pattern, and it causes occurrence of distortion.
On the other hand, when a wood-derived material consisting of wood flour and a binder of a thermosetting resin is used by the procedure with heating under pressure, there evolves the following problems. Thus, as the first problem, the presence of wood flour lowers heat conductivity so that hardening of the resin takes place only to 25 millimeter deep from the surface at the best. Therefore, the limit of the thickness of mold to be produced is 50 millimeter and when it is thicker than this limit the internal part deeper than 25 millimeter from the surface is not hardened remaining in a powdery state. (This is due to that containing more than 30% of wood flour lowers heat conductivity and that heat required by hardening does not reach the internal part.) Additional heating of the material in a dielectric heat dryer for hardening of this internal part should cause evaporation of moisture from this part, but the outer resin that has already been hardened does not pass the moisture with the result of swelling and distortion of whole material. The second problem is that due to elevation of temperature from the outer part during heating causing gathering of resin to the surface, the resin density at the contacting surface between the material and forming die becomes higher and subsequently the homogeneity of the outer and internal parts of the material is damaged. From these 2 reasons the wood-derived materials using thermosetting reins as binder are not homogeneous and inadequate for use as the parent material for wooden patterns.